Boilers use gas or oil to combustion to create heat and generate steam or hot water. The steam or hot water is used to carry the energy of the boiler throughout a facility for use in process or comfort heating. In essence, boiler maintenance determines your unit’s efficiency. You can enjoy lower bills with a highly efficient boiler, especially if you understand how to adjust the boiler operating parameters and employ effective heat recovery strategies to enhance efficiency.
What is a Good Boiler Efficiency?
Efficiency is simply the ratio of the energy output divided by the energy input into any system. Boilers have various subsystems that influence their overall efficiency. The types of efficiencies determining a boiler’s usefulness are:
Thermal efficiency considers the ability of the boiler’s heat exchanger to transport heat energy to the waterside. Soot and scale buildup in the tubes affect thermal efficiency. This will present itself as increased stack temperatures.
Combustion efficiency considers the ability of the burner to burn fuel completely, converting it to thermal energy. Lower combustion efficiency can be caused by excess air, though the combustion is complete, the excess Oxygen (and Nitrogen and other constituents of air) carry heat away from the boiler and out of the stack – this will be visible to you as excess stack temperatures or increased Oxygen levels in the flue gas if you use a combustion analyzer. Lower efficiency can also be caused by incomplete combustion, this will present itself as stack CO (Carbon-Monoxide), heat transfer surface fouling, pollution, lower combustion efficiency, flame instability, and a potential for explosion or backfire.
Fuel-to-Steam Efficiency takes both of the above into account but also looks at other components of boiler efficiency such as shell losses (i.e., Radiative and Convective Losses) and blowdown losses. This may also be called “in-service” efficiency.
If you are comparing efficiencies between different boilers (or really, any pieces of equipment), be sure you are comparing apples-to-apples with regards to the types of losses being considered as the verbiage used may differ slightly between manufacturers.
Boiler Efficiency Formulas
Boiler efficiency is the ratio of the useful heat output to the cumulative energy input. It is calculated as follows η=Qabs/Qin, this is also known as the input-output method, where:
- η represents boiler efficiency
- Qabs is the amount of useful heat absorbed
- Qin represents the heat and energy input into the boiler
This method requires precise measurement of the energy input and output/lost from the boiler. A more common method is known as the “by-losses” method, where you start with 100% less the measured losses, i.e. 100% – Stack Losses – Radiative Losses – Convective Losses – etc. = Efficiency (the losses included depend on the type of efficiency under consideration). Stack losses are read from a combustion analyzer (which also takes into account combustion efficiency). Radiative and Convective loss data should be available from the boiler manufacturer.
Possible Boiler efficiency depends on the type of boiler, fuel burned, steam pressure or water temperature, and other factors. In general, for steam boilers, It ranges from 80-8488% (before the use of heat recovery equipment, and 82-95+% efficiency for hot water boilers (from non-condensing to condensing efficiencies).
Do Boilers Lose Efficiency with Age?
Boilers are long-lasting and can last for many years to decades or more. However, the boiler’s efficiency levels reduce over time as the boiler ages. This reduction can be caused by scaling/fouling of the heat transfer surfaces, outdated controls, worn-out linkages on the fuel train(s) and air damper, or failing internal components in the boiler.
Many of these issues can be overcome with proper maintenance, repairs, and/or upgrades, but some older designs are just not as efficient as modern boilers. This fact, combined with aging equipment, makes it a good idea to evaluate replacing aging boilers, by looking at accurate payback estimates (Return on Investment, ROI) and the life cycle cost of the more modern equipment versus your existing equipment.
There are many types of boilers to consider and with these there are numerous control and heat recovery options available to maximize efficiency and reduce energy costs. For larger commercial and industrial boilers, the fuel bill will quickly outstrip the purchase price of the equipment. Small improvements in efficiency can result in very notable savings!
What Affects Boiler Efficiency?
Simply put, anything that increases a boiler’s stack temperature reduces efficiency, and anything that reduces stack temperature increases efficiency.
What Reduces Efficiency:
- Scale/Soot on the boiler’s heat exchanger surfaces reduces heat transfer to the water and increases stack temperature.
- Excess Stack Oxygen also increases stack temperature
- Insufficient Combustion air reduces efficiency by not completely converting fuel to heat.
- Older, inefficient boiler designs that do not effectively transfer the burner’s heat into the boiler’s water.
What Increases Efficiency:
- Proper annual maintenance, cleaning the boiler’s heating surfaces.
- Routine preventative maintenance and tune-ups to keep boiler and burner components in good condition and more frequent tune-ups optimizes combustion efficiency and reduces the chance for soot to form.
- Control and/or burner upgrades to more precisely control combustion.
- Heat Recovery Equipment such as Economizers (Feedwater Heaters) or Continuous Blowdown Heat Recovery systems to absorb otherwise wasted heat.
How To Maximize Efficiency and Reduce the Operating Costs of a Boiler
You can apply several tips to optimize your boiler’s efficiency. These include:
1. Lowering the Stack Temperature
For idle boilers, reduce operating pressure in steam boilers and the operating temperature for hydronic boilers. Consider a day or night setback to lower the stack temperatures.
2. Fitting an Economizer
As your boiler operates, it discards hot flue gas. This otherwise wasted heat can be useful for heating the feed water to the boiler, or other hot water needs in your facility. Installing an economizer saves fuel and guards your unit against the effects of a cold water supply. Economizers are available in a variety of configurations, including dual exchanger units where one circuit traditionally heats feedwater and the other heats an additional water source – these can even be designed to help steam boilers reach condensing efficiencies.
3. Tuning the Burner
The amount of air can affect your boiler efficiency. Sufficient air is necessary for adequate fuel combustion in the boiler, but excess air reduces efficiency – a proper air-fuel ratio is one key to peak efficiency.
4. Cleaning the Fireside to Alleviate Soot
Over time, your boiler tubes may accumulate soot from the fireside. The soot coating acts as an insulator leading to slow heat transfer. This leads to more fuel consumption. Moreover, the hot gas will not heat your water due to low heat transfer. Inspect your boiler tubes regularly and schedule regular maintenance to increase boiler efficiency.
5. Cleaning the Waterside
Water treatment keeps your boiler’s waterside clean and prevents leaks. Inspect the waterside often and clean mud drums. Improper water chemical use and failure to blow your boiler regularly can cause scale to build up and hinder heat transfer. This affects your boiler’s efficiency greatly.
Get Help From W.C. Rouse & Son
Are you looking for boiler equipment and services? Then, W.C. Rouse & Son is your perfect consultant. We are boiler room experts with over 70 years’ experience. Contact the W.C. Rouse team for all your boiler needs.